1. Field of the Invention
The present invention relates to a mobile communication system, a wireless base station, a radio network controller, and a power control method.
2. Description of the Related Art
In a mobile communication system, generally, multipath fading or the like causes instantaneous level fluctuations on a receiver side, sometimes drastically degrading reception characteristics of uplink in a wireless base station or reception characteristics of downlink in a mobile station. As a technology to reduce such degradation, in a mobile communication system using CDMA, transmission power control is individually performed for uplink and downlink.
Hereinafter, an outline of the transmission power control in uplink is shown (for example, see 3GPP, “TS25.211 V5.6.0 Physical channels and mapping of transport channels onto physical channels (FDD)” and 3GPP, “TS25.214 V5.9.0 Physical layer procedures (FDD)”). FIG. 1 shows a configuration of transmission power control loops based on SIR measurement. The transmission power control includes two loops of inner and outer loops. In the inner loop, SIR of a received signal at each slot is measured by a SIR measurement unit 17. Transmission power control (TPC) bits to control an increase and a decrease of transmission power are created in a TPC command creation unit 19 so that the measured SIR value is equal to a target SIR value. The created TPC bits are transmitted on a dedicated physical control channel DPCCH in paired link (downlink when controlling the transmission power in uplink). On the other hand, the same target SIR value does not always provide same reception quality (BLER or BER) depending on differences in the number of propagation paths, a propagation environment such as moving speed of the mobile station (maximum Doppler frequency), and the SIR measurement method. Accordingly, in a BLER measurement unit 14, the reception quality is measured through the outer loop for a longer interval, and the target SIR value is corrected in a target SIR correction value creation unit 16 in long cycles based on the measured reception quality. For example, when the outer loop control is performed based on BLER, BLER is measured based on the number of transport blocks in a data series whose calculated CRC results are correct after error correction decoding, and a correction value of the target SIR value is obtained so that this measured BLER value is equal to a required BLER value.
Herein, FIG. 2 shows a frame structure of dedicated physical channel DPCH composed of DPCCH as a downlink dedicated physical channel for notification of the transmission power control bits and DPDCH as a dedicated physical data channel. A portion indicated by “TPC” includes the aforementioned transmission power control bits. A portion indicated by “TFCI” is called a transport format combination indicator and includes information indicating a transport format combination. A portion indicated by “Pilot” includes dedicated pilot bits. The power of these three pieces of information, i.e. TPC bits, TFCI bits, and Pilot bits, is set to the value which is equal to “DPDCH power+power offset”. The power-offset values are signaled through an upper layer.
When the mobile station communicates with a plurality of wireless base stations, the plurality of wireless base stations individually perform the transmission power control and send transmission control bit patterns using the downlink DPCCH. At this time, in uplink, the reception quality of at least one of the above plurality of wireless base stations should meet required quality. Accordingly, the mobile station reduces the transmission power when at least one of the transmission power control bit patterns of the downlink indicates a power reduction command to reduce power. The mobile station increases the transmission power only when all the transmission power control bit patterns indicate a power increase command to increase power.
As described above, generally, in the case of communicating with a plurality of wireless base stations, the mobile station increases the transmission power only when each of the transmission power control bit patterns from the above plurality of wireless base stations indicates the power increase command and reduces the transmission power when any one of the transmission power control bit patterns indicates the power reduction command. Herein, there are no problems when an error rate of the transmission power control bits is 0%. However, actually, the error rate of the transmission power control bits is not 0%, and errors could occur. In this case, when any one of the power increase commands is mistaken for the power reduction command, the mobile station reduces the transmission power. The transmission power of the mobile station tends to be smaller than the required transmission power as a whole. This results in degradation in the uplink reception quality in the wireless base stations.
In the light of the aforementioned problem, an object of the present invention is to provide a mobile communication system, a wireless base station, a radio network controller, and a power control method capable of reducing the error rate of the transmission power control bits and maintaining uplink reception quality of each wireless base station.